In recent years, the search for cost-effective production of complex ceramic shapes used at elevated temperature has stimulated the research and development of polymeric ceramic precursors. Although significant progress has been made over the past years for fabrication of large complex injection molded shapes used in automotive engines, poor reliability and reproducibility have remained as critical issues, especially for large cross-section parts. The difficulty is caused by a fundamental problem related to uneven part shrinkage during binder burnout process. Therefore, the exploration of novel materials is needed to alleviate these problems.
Polycarbosilanes, polysilanes and polysilazanes have shown a wide range of application in ceramic processing. For example, hexaphenylcyclotrisilazane and methylphenylpolysilane were used to infiltrate porous reaction sintered silicon nitride, K. S. Mazdiyasni, R. West, and L. D. David, Am. Ceram. Soc., 61, (1978), pp. 504-508. Polycarbosilane and polysilazane have also been used to spin SiC, and Si.sub.3 N.sub.4 fibers, S. Yajima, Am. Ceram. Soc. Bull., 62, [8 ] (1983), pp. 893-898; C. L. Schilling, Jr., J. P. Wesson, and T. C. Williams, Am. Ceram. Soc. Bull., 62, [8 ] (1983) pp. 912-915; R. West, L. D. David, P. I. Djuovich, H. Yu, and R. Sinclair, Am. Ceram. Soc. Bull., 62, [8 ] (1983), pp. 899-903; and D. Seyferth and G. H. Wiseman, Am. Ceram. So., Comm., July 1984, pp. C-132-133. Most of the polymeric preceramic precursors are relatively stable in air and are chemically compatible to Si.sub.3 N.sub.4 and SiC powders. They show good flow characteristics and produce ceramic products upon pyrolysis. These precursors should be good binder candidates for injection molded Si.sub.3 N.sub.4 parts.
Several polycarbosilane precursors' routes to silicon carbide ceramic composition have been developed, see: U.S. Pat. Nos. 4,414,403; 4,472,591; and 4,497,787 to C. L. Schilling, Jr., et al., and T. C. Williams, Am. Chem. Soc., Polymer Reprints, 25, (1), 1, (1984). The effectiveness of these materials as ceramic precursors is derived from a common structure feature, backbone bracketing at silicon atoms, generated either during synthesis or during conversion to ceramics. Examples of these polymer types are: hydrosilyl-modified polycarbosilanes, polysilahydrocarbons, and vinylic. Polysilanes are all prepared via active metal dechlorination of silane monomers using either potassium or sodium. Reactivity differences between the two metals have been a critical factor in the preparation of these polymers which typically provide 50-60 wt % yields of SiC compositions when pyrolyzed in an inert atmosphere. Thermoplastic polymers can be prepared with high yield through potassium metal dechlorination of mixtures of (CH.sub.3).sub.3 SiCl, CH.sub.3 SiHCl.sub.2, and CH.sub.2 .dbd.CHSiCH.sub.3 Cl.sub.2 in tetrahydrofuran solvent. Potassium causes disilylation of CH.sub.2 .dbd.CHSi.tbd. groups to form linear polycarbosilane. While a thermoset polymeric product with a formulation of ##STR1## forms when dechlorinated with sodium metal. The vinyl --CH.dbd.CH.sub.2 and SiH groups of vinylic polysilane provide an efficient thermal cross-linking mechanism not involving oxygen or weight loss. Cross-linking occurs via a combination of vinyl and SiH addition, and vinyl polymerization. Because of its advantages in cost, safety and precursor performance, the thermoset polycarbosilane is the preferred polymeric precursor.
The colorless, viscous liquid polymer resulted from dechloronation of 0.85 moles/0.3 moles/1.0 moles of (CH.sub.3).sub.3 SiCl/(CH.sub.3)SiHCl.sub.2 /CH.sub.2 .dbd.CHSiCH.sub.3 Cl.sub.2 with sodium metal begins to thermoset from 75.degree. to 100.degree. C., at which point pyrolytic degradation commences. Thermogravimetric Analysis (TGA) of this polymer in nitrogen indicates that 8-10% of weight loss occurs in the range of 100.degree. to 175.degree. C. The ceramic yield is in the range of 60 to 65 wt % upon pyrolysis at 1000.degree. C. The volatile portion of the polymer, identified as low molecular weight silanes by gas chromatograph/mass spectrometry, can be removed by vacuum distillation at 65.degree.-70.degree. C., 10.sup.-3 mm-Hg. After removal of voltiles, this polymer thermosets in the range of 110.degree. to 135.degree. C. Because of its thermal cross-link nature, polycarbosilane provides several limitations as a binder for injection molding. It was partially cross-linked during compounding and completely hardened in the injection molders' chamber. The rate of cross linking was enhanced by the injection speed, molding pressure and injection barrel temperature. Therefore, the development of an inhibitor or a retarder to delay the thermosetting which prevents premature cross-linking is essential.